packed-refs: use `fwrite()` when passing refs verbatim

The `write_with_updates()` function uses a `struct ref_iterator` to
iterate over all refs to write to the temporary packed-refs file. It
receives the iterator from `packed_ref_iterator_begin()` which takes a
snapshot of the 'packed-refs' file.

While writing to the new packed-refs file, writes are routed via
`write_packed_entry()` which uses `fprintf()`. Even for references which
haven't changed, we use the same mechanism. This is slow since we need
to do string parsing and formatting. Instead, let's track the position
of unchanged references in the snapshot iterator and directly write
verbatim via `fwrite()`.

With this, any sanitization which was happening as a side effect of
reformatting is now lost. But that was never the job of this section of
the code, since the main intention is to simply rewrite the remaining
refs post deletion of the selective few.

This removes the unnecessary formatting operation involved. We can see a
consistent ~20% performance improvement when deleting from packed
references.

Benchmark 1: update-ref: delete ref (refcount = 100000, revision = master)
  Time (mean ± σ):      28.7 ms ±   1.7 ms    [User: 22.5 ms, System: 5.9 ms]
  Range (min … max):    26.7 ms …  33.3 ms    46 runs

Benchmark 2: update-ref: delete ref (refcount = 100000, revision = b4/kn-speedup-packed-refs)
  Time (mean ± σ):      23.8 ms ±   1.2 ms    [User: 17.5 ms, System: 6.0 ms]
  Range (min … max):    22.1 ms …  27.7 ms    56 runs

Summary
  update-ref: delete ref (refcount = 100000, revision = b4/kn-speedup-packed-refs) ran
    1.21 ± 0.09 times faster than update-ref: delete ref (refcount = 100000, revision = master)

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>